It is known that in the field of medicine there are substrates which are coated with high viscosity materials. For certain purposes it is sensible that these coatings do not generate a sealed surface but are applied as dots, which for instance allows sweat and other elimination products to escape from skin under bandages and not cause maceration. An adequate method of achieving this dotted coating is offered by rotational screen extrusion.
In this method a rotating screen has a nozzle located inside it, and through the nozzle the liquid that is to be applied is brought from outside to inside the screen. It is then extruded out through the holes in the screen in the direction of the substrate that is to be coated. Dependent on the substrate transport speed (rotational speed of the screen drum), the screen is lifted up by the substrate. Depending on the adhesion and internal cohesion of the liquid, the slugs which have already swelled so as to adhere to the supporting material draw out the limited stock of hot melt adhesive in the hole to a sharp contour, assisted by the sustained extrusion pressure, on to the supporting material.
On completion of this transport there forms, depending on the rheology of the liquid, over the pre-determined basis area a more or less heavily crumpled domed surface of the slug. The height to base ratio of the slug depends on the hole diameter to drum screen wall thickness ratio, and on the physical characteristics (flow behavior, surface tension and wetting angle on the supporting material) of the liquid.
Regarding substrate materials many types are prescribed and have been used in practice, including films, woven fabrics, knitted textiles, fleeces, gels and foams. In the medical sector there are particular requirements for the supporting materials. The materials must be compatible with the skin, generally permeable to air and/or water, easily formed and ductile. Based on these requirements, often the thinnest and weakest supporting material is preferred. For handling and use the supporting material must however be sufficiently strong and if necessary have only a limited tendency to stretch. Furthermore the supporting material should exhibit sufficient strength and limited tendency to stretch, even when wet through. It is known within the textile industry that partial coatings can be transferred. EP 0 675 183 A1 describes a method for transferring melt adhesive geometries on to a specially cross-linked substrate. In EP 0 356 777 A1 also mentions a non-adhering intermediate support medium. Also the use of a coated counter-pressure roller as auxiliary medium for the transfer is described (CH 648 497 A5), although here the process does not refer to self-adhesive products.
The transfer especially of high viscosity liquids such as thermoplastics from the counter-pressure roller on to an introduced supporting material is subject under current technology using thermo-screen extrusion to very severe restrictions, because the plastic-coated flexible counter-pressure rollers have coating thicknesses in the range 10 to 20 mm, the thermal insulation effect of which makes it difficult to cool down the thermoplastic on the circumference of the roller. This means that the cohesion of the slugs necessary to overcome the adhesion to the roller can be attained only at very low process speeds.
The arrangement of nozzle and screen are described in essentials in CH 648 497 A5, improvements to the method are described in EP 0 288 541 A1, EP 0 565 133 A1, EP 0 384 278 A1 and DE 42 31 743 A1.
The method requires that the liquid be applied with a pressure sufficiently high for extrusion from the screen holes. Pressures of the order of between 5 and 30 bar are necessary for this, depending on the hole diameter of the screen, the viscosity of the liquid and the processing speed. Until now the method has been to generate this pressure by upstream devices. The consequence of this is that the entire nozzle area including even the rotary seals at the sides of the rotating assembly are also subjected to this high pressure. The seal is provided by flexible lip seals (as wiper blades in CH 648 497 A5) or by nozzle lips adapted to the curvature of the screen profile (EP 0 565 133; U.S. Pat. No. 5,122,219).
Furthermore the counter-pressure rollers used are exclusively plastic-coated flexible type (CH 648 497 A5, EP 0 565 133 A1).
The transfer described above of especially of high viscosity liquids such as thermoplastics from the counter-pressure roller on to an introduced supporting material is subject under current technology using thermo-screen extrusion to very severe restrictions, because the plastic-coated flexible counter-pressure rollers have coating thicknesses in the range 10 to 20 mm, the thermal insulation effect of which makes it difficult to cool down the thermoplastic on the circumference of the roller. This means that the cohesion of the slugs necessary to overcome the adhesion to the roller can be attained only at very low process speeds.
A particularly advantageous process feature has until now been seen to be that the center-to-center spacing between screen and counter-pressure roller should be maintained at less than the total of the radii (EP 0 565 133 A1, DE 42 31 743 A1), so that positive pressure is generated.
There is a limit to the force between the rotating screen at the sealing zone of the nozzle and the counter-pressure roller, since excessive force will cause the risk of mechanical damage or excessive wear, so that the liquid pressures necessary for extrusion coating lead in practice to the well known problems of leakage in the nozzle/screen system.